Form inside a form, is that alright?

Understanding form submission

The HTML5 specification explicitly bans nested forms. This regulation stems from the need to ensure that each <form> is individually responsible for performing its own submission process. Nesting has the potential to create data conflicts and submission ambiguities.

Say, the data from the inner form doesn't get serialized appropriately, therefore resulting in missing or unauthorized form data being sent to the server.

Dynamic structures and JavaScript

Understanding that you need to handle complex form logic, you should consider using JavaScript. If you need to mimic the behavior of nested forms, you could:

1. Characterize inputs using <div> or <fieldset> elements.
2. Deploy JavaScript to dynamically process data and submit it with ajax or through a single global form.

This provides flexibility without violating HTML rules:

//Time to spy on the parent...form
document.getElementById('parentForm').addEventListener('submit', function(e) {
  e.preventDefault();
  //Do not go gentle into that good night, rage, rage against the dying of the light
  var payload = {
    field1: document.querySelector('[name="field1"]').value,
    //Don't forget me! Add other field values here
  };
  // Mission Possible: AJAX submission logic here
});
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Form attribute for complex associations

The form attribute in HTML5 provides a way to associate input elements with multiple forms. This is particularly handy when placing inputs outside their form:

<form id="mainForm"></form>

<input type="text" name="outsideField" form="mainForm" />
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With this arrangement, the input will be submitted along with the mainForm data even though it's not a direct child within the DOM structure.

Handling nested interaction scenarios

When dealing with portions that call for isolated validation or interactivity, you can:

1. Allocate distinct IDs to each form.
2. Utilize the form attribute to tie input fields outside the form.
3. Implement JavaScript for manipulating these elements and imposing a single submission endpoint.

Accessibility and UX considerations

When talking about accessibility, the key is ensuring that forms are organized coherently. This entails labeling form controls explicitly and evading complex structures which confuse users, particularly those who rely on screen readers.

Form submission expectations not met adversely affects the user experience (UX). Nested forms frequently produce unpredictable outcomes, thus leading to user dissatisfaction and reduced trust in the application.

Browser compatibility and semantic web

The HTML5 'form' attribute is widely supported across up-to-date browsers. However, older versions like IE11 might require extra polyfills or fallbacks to sustain compatibility.

Furthermore, the correct usage of semantic HTML ensures accessibility and improves SEO and machine readability. By employing well-structured forms, you make a crucial contribution to a cleaner, more semantic web.